Our invention relates to image sensors, and more particularly to an image sensor of the kind employing photodiodes as photoelectric converters to be electrically scanned sequentially for providing electric signals representative of optical energy incident thereon.
The image sensor has been known which comprises photoelectric converters for translating optical energy into electric energy, and analog switches for electrically "scanning" the photoelectric converters in order to cause the same to produce electric signals. Usually, the analog switches have taken the form of field effect transistors (FETs) disposed adjacent the photoelectric converters.
In an image sensor of this conventional type fabricated in the form of an integrated circuit, each FET has had to be sized to fit the width (e.g. 125 micrometers) of one photoelectric converter, that is, of one picture element. It has been no easy task to form such microminiaturized FETs. An additional difficulty has arisen as the three wiring conductors of minimal widths (e.g. 10 micrometers) for the drain, source and gate of each FET has had to be formed on prescribed parts of the baseplate. The image sensors of this type have therefore been very expensive because of the noted difficulties in manufacture.
Patent Cooperation Treaty International Publication No. WO 90/1846 represents a conventional solution to this problem. It teaches the use of a serial circuit of diodes for scanning photoelectric converters in the form of photodiodes. The resulting image sensor is far easier of manufacture as the required wiring conductors can each be as wide as 20 micrometers or so.
We have, however, found a weakness in this prior art image sensor. When the image sensor is supplied with a sawtooth voltage for scanning the photodiodes, the scanning voltages for the photodiodes have not saturate but continued to increase with time beyond the desired saturation points. Such excursions of the scanning voltages are undesirable as they deteriorate the response of the image sensor to optical input of low intensity (e.g. 10 lux) in particular. We will elaborate on this weakness of the prior art image sensor in the course of the detailed description of our invention.